The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention relates to improved techniques for ascertaining the end of an etch process for endpointing purposes while etching through a selected layer on a substrate.
In the manufacture of semiconductor devices, such as integrated circuits or flat panel displays, the substrate (e.g., the wafer or the glass panel) may be processed in a plasma processing chamber. Processing may include the deposition of layers of materials on the substrate and the selective etching of the deposited layer(s). To prepare a layer for etching, the substrate surface is typically masked with an appropriate photoresist or hard mask. During etching, a plasma is formed from the appropriate etchant source gas to etch through regions unprotected by the mask. The etching is terminated once it is determined that the target layer is etched through. This termination of the etch is typically referred to as the etch "endpoint."
To determine when to terminate an etch, many techniques have been employed in the art. By way of example, the etch may be terminated upon the expiration of a predefined period of time. The predefined period of time may be empirically determined in advance by etching a few sample substrates prior to the production run. However, there is no allowance made for substrate-to-substrate variations as there is no feedback control.
More commonly, the end of an etch process may be dynamically ascertained by monitoring the optical emission of the plasma. When the target layer is etched through, the optical emission of the plasma may change due to the reduced concentration of the etch byproducts, the increased concentration of the etchants, the increased concentration of the byproducts formed by reaction with the material(s) of the underlayer, and/or due to the change in the impedance of the plasma itself.
It has been found, however, that the optical emission-based technique has some disadvantages. By way of example, the use of some etchants and/or additive gases interferes with the optical emission endpoint technique, giving rise to inaccurate readings. As a further example, as the feature sizes decrease, the amount of film exposed to the plasma through openings in the mask is also reduced. Accordingly, the amount of byproduct gases that is formed from reactions with the exposed film reduces, rendering signals that rely on plasma optical emission less reliable.
It has been found that, as the target layer etch is completed and the underlayer is exposed to the plasma, the self-induced bias of the substrate may change. By way of example, for the etch of a dielectric target layer, the self-induced bias of the substrate is observed to change as a conductive underlayer is exposed to the plasma. As a further example, for the etch of a conductive target layer, the self-induced bias of the substrate is observed to change when a dielectric underlayer is exposed to the plasma. By monitoring the change in the self-induced bias of the substrate, the end of the etch process may be ascertained for endpointing purposes.
To facilitate discussion, FIG. 1 illustrates a typical endpointing arrangement wherein the self-induced bias on the wafer is monitored to determine when the target layer is etched through for the purpose of endpointing the etch. As shown in FIG. 1, a wafer 102 is shown disposed on an electrode 104, which is typically made of a metallic material. Electrode 104, which functions as a chuck in this example, is energized by an RF power source 106 through a capacitor 108. During etching, the self-induced bias on wafer 102 is detected at a node 110 through a monitoring circuit 112. Monitoring circuit 112 include a low pass filter 114, which blocks the RF component of the signal and allows only the DC component to pass through. Since the self-induced bias on the wafer tends to be in the hundreds of volts, the signal that is passed through low pass filter 114 is typically stepped down through a voltage divider circuit to allow the monitoring electronics (not shown to simplify the discussion) to monitor the change in the self-induced bias on wafer 102. This information pertaining to changes in the self-induced bias on the wafer allows the endpointing electronics to determine when the etch should be terminated.
However, the sensitivity and accuracy of the monitoring technique discussed in FIG. 1 may degrade as the percentage of the target film exposed to the plasma decreases and/or if the DC conductivity between the plasma and the electrode is decreased (e.g., due to the presence of a dielectric layer underlying the target layer to be etched). Furthermore, the monitoring technique of FIG. 1 is typically ineffective when electrostatic chucks are employed. This is because electrostatic chucks typically employ a dielectric layer between the conductive chuck body and the substrate. The presence of this dielectric layer interferes with the current path between the plasma and the chuck, rendering it very difficult to accurately determine the self-induced bias on the wafer at node 110. Furthermore, the relationship between the voltage detected at node 110 and the self-induced bias on wafer 102 is not linear. By way of example, the resistance of the electrostatic chuck depends, in part, on the voltage existing on the chuck. Accordingly even if a signal can be detected at node 110, it is difficult to correlate the signal detected with the self-induced bias on the substrate for endpointing purposes.
In view of the foregoing, there are desired improved techniques for detecting the end of a plasma etch process for endpointing purposes.